The crowd cleared and a group of men, naked from the waist up, stepped forward and began beating their chests. As the chanting grew louder, they unwrapped bundles of steel knives attached to chains and began beating themselves on their backs, shouting "Hussain, Hussain".

The blood-soaked ceremony is an annual ritual for Shia Muslims across the world, but in Pakistan this year it comes at a time of renewed sectarian tension.

In a string of violent outbreaks between the Sunni community and the minority Shias - who make up 20% of Pakistan's 140m population - more than 50 people have been killed in the past two months.

In Karachi on Wednesday, a bus driver found a bomb under his front seat and managed to defuse it. A Shia procession was due to pass by minutes later.

Just 24 hours earlier, a former leader of a Shia Muslim group was shot dead by gunmen on a motorbike, near Multan, in the centre of the country.

Reports say that there are up to 10,000 trained sectarian terrorists in Pakistan, supplied with arms from caches in mosques across the country.

Riot police and heavily armed soldiers are now patrolling Rawalpindi and other cities across the country in a bid to prevent further attacks. In Lahore alone, 10,000 soldiers were on duty for the Shia procession - the country's biggest.

The procession of mourners dressed in black shalwar kameez suits commemorates the martyrdom of Imam Hussain, the grandson of the prophet Mohammad, who died in 680AD in Karbala, in modern-day Iraq.

The Ashura ceremony is the culmination of 10 days of mourning in Muharram, the first month of the Muslim year.

But for all the recent upsurge in violence, most in the crowd at Rawalpindi appeared unconcerned by the threat.

"This day is most sacred for us. We are ready to die for this day," said Ahmed Ali, a 26-year-old web page designer from Islamabad.

"Things are becoming difficult now. The other sects are very organised and they think if they kill us they will go straight to heaven. But I don't think most people in Pakistan agree with them. For the past 50 years, we have lived peacefully in Pakistan. Why can't that continue?"

As the men beat deep cuts into their backs, women stood on the side of the road, dressed in black veils, beating their chests with the rest of the crowd.

At one point, a white horse - signifying that which Hussain rode in battle - was led into the centre of the crowd, and hands reached out to touch the animal.

Many of the men, already carrying scars on their backs from last year, came away with bloody wounds needing medical treatment. The knives ranged in size from just a few inches long to over a foot, and several young boys had joined in the self-flagellation.

Every few minutes, when a mourner's injuries appeared to become too serious, the crowd would push forward and wrestle the blades from his hand.

Across the border in Afghanistan, the hardline Sunni Muslim Taliban this week allowed Shias - again in the minority - to mark Ashura for the first time since they seized power nearly five years ago.

Many of Pakistan's most wanted Sunni sectarian terrorists are believed to be in hiding in Afghanistan, protected by their close links to the Taliban.